Speed and voltage control means for an alternating-current motor, particularly a polyphase induction motor

ABSTRACT

The invention relates to a speed and voltage control system for an alternating current motor such as a polyphase induction motor. The system includes voltage and frequency regulators and an inverter. In order to avoid the damaging effects of reverse currents and excessive voltages due to hyper-synchronous speed operation upon reducing motor speed, the direct current voltage is made the independent variable with the control frequency being dependent on the actual direct current voltage at the inverter.

United States Patent Jensen [54] SPEED AND VOLTAGE CONTROL MEANS FOR ANALTERNATING- CURRENT MOTOR, PARTICULARLY A POLYPHASE INDUCTION MOTOR[72] Inventor: Arne Jensen, Nordborg, Denmark [73] Assignee: DanfossA/S, Nordborg, Denmark [22] Filed: Feb. 18, 1971 [2]] Appl. No.: 116,657

Related US. Application Data [63] Continuation of Ser. No. 806,655,March 12, 1969,

abandoned.

s2 U.S.Cl ..3l8/227,3l8/230,3l8/23l,

318/345 51 Int. Cl. ..H02p 5/40 58 FieldofSearch ..318/227-23l,345

VOLTAGE REGULATQRW [151 3,670,224 June 13, 1972 [56] References CitedUNITED STATES PATENTS 3,344,326 9/1967 Risberg ..3l8/23I X 3,500,1583/1970 Landau et al ..3l8/227 Primary Examiner-Gene Z. RubinsonAttorney-Wayne 13. Easton ABSTRACT The invention relates to a speed andvoltage control system for an alternating current motor such as apolyphase induction motor. The system includes voltage and frequencyregulators and an inverter. In order to avoid the damaging effects ofreverse currents and excessive voltages due to hypersynchronous speedoperation upon reducing motor speed, the direct current voltage is madethe independent variable with the control frequency being dependent onthe actual direct current voltage at the inverter. 7

2 Claims, 7 Drawing Figures FREQUENCY REGULATOR IMPULSE 23 GENERATORQUENCHING CIRCUIT FATENTED l??? 3 670.224

' sum 20F 2 VOLTAGE FREQUENCY REGLHJTQRI REGULATOR IMPULSE 2| :23GENERATOR SPEED AND VOLTAGE CONTROL MEANS FOR AN ALTERNATING-CURRENTMOTOR, PARTICULARLY A POLYPHASE INDUCTION MOTOR This is a continuationof Ser. No. 806,655, filed Mar. 12, 1969, and now abandoned.

The invention relates to a speed and voltage control means for analternating-current motor, particularly a polyphase induction motor, inwhich means a regulable direct-current voltage is'fed through anintermediate circuit containing at least one condenser to an inverterwhich comprises at least two branches having two controllable rectifiersarranged in series and free-running diodes associated therewith, themotor speed or the frequency of the control impulses sent to theinverter and the motor voltage being regulated in a predeterminedrelationship to each other.

In order to control the speed of the motor, the frequency of the voltagesupplied to the motor or the frequency of the control impulses sent tothe inverter is altered. Raising of the frequency, however, leads to anincrease in the inductive resistance. In order to keep the magnetizationof the motor at the corresponding level, it is therefore necessary toraise the voltage. Usually, provision is made for the voltage to followthe frequency in a linear manner. I

Inverter switching arrangements are known wherein an alternating-currentvoltage is rectified, regulated as to value and finally smoothed bymeans of an intermediate circuit which consists of a series choke and ashunt condenser and then inverted again to AC. The free-running diodesare for the purpose of passing currents that still flow, after thecontrollable rectifiers have been cut out, due to the inductancespresent.

A disadvantage occurs, however, in control switching arrangements ofthis kind. If, for the purpose of reducing the motor speed, thefrequency of the control impulses is reduced and, in dependence thereon,the voltage fed to the motor is also reduced, the motor does notimmediately respond to this signal because of its mass inertia. Instead,its speed drops slowly. This means that it runs for a time at anhypersynchronous speedin relation to the newly adjusted frequency-andtherefore operates as a generator. This generator therefore feeds acurrent, through the free-running diodes, into the intermediate circuitwhere it charges the condenser or condensers. Since the intermediatecircuit has been regulated, with frequency, to a lower voltage, thiscurrent can assume very high values. The charging of the condenser leadsto excess voltages. Both of these factors can have very harmful effectsupon the components of the inverter or of the voltage supply system.

The object of the invention therefore is to provide a speed and voltagecontrol means of the initially described kind, wherein the risk ofreverse currents and excess voltages is considerably smaller.

According to the invention, this object is achieved by controlling thenominal value of the direct-current voltage as an independent variableand the frequency in dependence upon the actual value of thedirect-current voltage at the condenser or the inverter.

By this procedure, the required speed is set with the help of thevoltage. This does not involve any disadvantage in normal operation,since the actual value of the voltage follows the nominal voltage thathas been set, and therefore the frequency, dependent upon the actualvalue, follows the setting for the nominal value of the voltage. On theother hand, considerable advantages are obtained if the nominal voltagevalue is reduced for the purpose of lowering the speed. Although themotor operates as a generator in this case too, the frequency of thecontrol impulses, however, does not follow the nominal value setting,but remains linked to the actual value for voltage, which is largelydetermined by the generator voltage. Consequently, the generatorinrelation to the frequency of the direct-current voltage fed to it-runsat a speed that is only slightly above the synchronous speed. Therefore,only very small currents flow and these can be accepted withoutdifficulty by the condenser in the intermediate circuit. The speed ofthe electrical machine diminishes gradually on the basis of the internallosses and the losses in the connected consumer unit. The actual valuefor voltage (and thus the frequency) follows the fall in speed until thenominal voltage value that has been set is reached.

Theoretically, the disturbing increase in' voltage due to the chargingof the condenser in the intennediate'circuit could also be eliminated byusing a condenser of correspondingly great capacity. Such a condenserwould, however, be large and costly. On the other hand, in a furtherform of the invention, the problem can be dealt with using very smalland inexpensive condensers if the voltage is regulated in such a waythat impulses, the width of which depends upon the nominal voltage valuethat has been set, are derived from a constant direct-current voltage bymeans of an electronic switch and are smoothed by means of theintermediate circuit, the impulse frequency being at least ten times,and preferably twenty times, as great as the mains frequency. Smallcondensers suffice for smoothing these impulses of relatively highfrequency, which condensers would, however, lead to very high excessvoltages at the inverter input point if the control relationship of theinvention were not used.

An advantageous form of the frequency regulator is a switchingarrangement wherein the voltage at the inverted converter controls atransistor, through which a condenser is charged, in dependence uponwhich the impulses are produced by a Unijunction sweep generator. Inthis way, a directly linear relationship between frequency and theactual voltage value is obtained.

The invention will now be explained in more detail by reference to anembodiment described in the drawing, wherein:

FIG. 1 shows, in a graph, the relationship between voltage andfrequency, and between torque and speed in the switching arrangementshere considered,

FIG. 2 shows frequency, voltage and current, plotted against time, inthe known control switching means,

FIG. 3 shows frequency, voltage and current plotted against time, in thecase of the switching arrangement of the invention,

FIG. 4 shows, in a schematic diagram, a form of arrangement of theswitching means of the invention,

FIG. 5 illustrates a form of construction of the electronic switch inthe voltage regulator,

FIG. 6 is a diagram illustrating the impulses passed by the electronicswitch, and

FIG. 7 shows an example of an arrangement for the frequency regulator.

In the graph seen in FIG. 1, the characteristic curve I shows therequired linear relationship between the voltage U at the input of theinverted converter and the frequency f of the control impulses fed tothe inverter. Curves 2 and 3 show the relationship between torque Md andspeed n in the case of a driven asynchronous motor. For each individualspeed, the working points are indicated by the point of intersectionwith a characteristic curve 4 dependent upon the consumer unit that itis required to power. If the frequency is regulated so that it dropsfrom f, to f the motor first continues to run at its initial speed. Inrelation to the frequency f now provided, the motor therefore operatesin hyper-synchronous range, i.e., as a generator, as shown by branch 3'of the curve.

FIG. 2 illustrates the course of the curves, with time, for such achange-over procedure. At the moment 2 the frequency is reduced from fto f, (curve 5). The current I here moves along the curve 6 into thegenerator range. The generator current charges the condenser in theintermediate circuit, so that the value U, for voltage at the input ofthe inverted converter rises in a pronounced manner, as indicated bycurve 7, before it drops again to the value U associated with the value1",.

According to the invention, and as seen in FIG. 3, it is not thefrequency but the nominal value U, that is displaced along the curve 8from the voltage value of U to U at the moment Consequently, the motorcurrent again moves along the curve 9 into the generator range, thegenerator current however, being considerably lower than in the caseillustrated in FIG. 2, since the frequency of the control impulses forthe inverter has held its initial value practically unchanged. Thisfrequency is controlled by the actual voltage value U, at the input ofthe inverter, this value decreasing in the manner shown by curve incorrespondence with the reduction in speed of the electrical machine.Accordingly, there is a change in frequency along the curve 11, Therequired new condition is reached at the moment 1 The time intervalbetween t and t depends upon how rapidly the machine expends energy,i.e., upon its internal losses and those of the associated consumerunit.

FIG. 4 shows a switching arrangement wherein the phases R, S and T of apolyphase current mains feed a rectifier 12 at the two output leads l3and 14 of which there is available a constant direct-current voltage.This is passed to a series voltage regulator 15 with which areassociated a battery-fed adjusting resistor 16, with a tapping 17 forfixing the nominal value, and a voltage divider, connected at the outputside thereof and comprising resistors 18 and 19, for tapping off theactual voltage value at the point 20. The main component of the voltageregulator 15 is an electronic switch which passes the constantdirect-current voltage only incrementally in impulse form, as describedin more detail below. On the output side of the voltage regulator isconnected an intermediate circuit, which consists of a series choke 21and a shunt condenser 22, these providing a smoothed mean value forvoltage from the impulses passed by the electronic switch.

The actual voltage value U, at the output of this intermediate circuitis passed by way of a short-circuit impedance 23 to a three-phaseinverter, which in the bridge arrangement 6 contains controllablerectifiers 24 and six free-running diodes 25 connected in antiparalleltherewith. The three output leads U, V and W feed a three-phaseasynchronous motor 26. The inverter also incorporates a common quenchingcircuit 27. The switching means for the inverter is to be understood asonly being represented schematically. It may be of any known design, canuse an individual instead of a common quenching system, can incorporatecommutating condensers, etc.

The same voltage U, is applied to the series control arrangementcomprising a resistor 28 and a frequency regulator 29. The latter sendsimpulses to a control device 30 which feeds the quenching circuit 27with impulses of a frequency six times as great as that required in thethree-phase supply system U, V and W and which supplies the controllablerectifiers 24 with striking impulses during the period in which they areto be conducting.

The voltage regulator 15 may have the switching arrangement illustratedin FIG. 5, for example. Contained in the lead 13 is a controllablerectifier 31. Connected in parallel with the latter are a diode 32, acontrollable quenching rectifier 33 and the series arrangementcomprising a recharging choke 34 and a commutating condenser 35. Afurther diode 36 is fitted between the quenching rectifier 33 and thediode 32. A control device 37 sends striking impulses to the twocontrollable rectifiers 31 and 33 in dependence upon the voltagesupplied through the tapping 17. When the rectifier 31 is blocked, thecondenser 35 becomes charged to the direct-current voltage madeavailable by the rectifier 12. When the rectifier 31 receives astrikingimpulse, it becomes conducting. This condition is stable until astriking impulse is sent to the rectifier 33. As soon as the latterbecomes conducting, the condenser 35 is recharged through the rechargingchoke 34 and produces an inverse voltage which acts by way of therectifier 31 and blocks it. At the same time, the quenching rectifier 33is also blocked. The condenser 35 can then be recharged again by way ofthe rectifiers 36 and 32 and, if required, recharged a little by therectifier 12, so that it is ready for use again.

The control device 37 sends striking impulses at regular time intervalsto the rectifier 31, so that the voltage impulses passed through thelead 13 are of predetermined frequency.

The striking impulses 33, however, are sent, in dependence upon therequired voltage, at different intervals b, c, d and c after thefirst-mentioned striking impulse, so that the voltage impulses passedthrough are of different widths, as shown in FIG. 6. The smoothingintermediate circuit 21, 22 is adapted to the repetition frequency ofthese impulses, e.g., 1000 cycles, so that a voltage representing themean value of the impulses can be picked up at the output of theintermediate circuit. The control device 37 may be of the form describedin U.S. patent application Ser. No. 792,684, filed Jan. 21, 1969, forexample.

FIG. 7 shows a form of the frequency regulator 29. A volt-- ageproportional to the actual voltage value U, is applied at the input 38.This voltage is passed to the base of the transistor 34 in the collectorcircuit of which is contained a condenser 40. This is charged at a ratethat is directly proportional to the voltage at the input 38. Thecondenser voltage is applied to a Unijunction transistor 41, the base 1of which is connected to the output 43 by way of a transformer 42, saidoutput leading to the control device 30. As soon as the voltage at thecondenser 40 has reached a prescribed threshhold value, the condenserdischarges through the Unijunction transistor 41, the latter sending animpulse. The sequence of impulses at the output 43 is thereforeproportional to the voltage at the input 38. The sequence of impulses isprocessed in the control device 30 in the manner required forcontrolling the inverter. An example of the form of the control deviceis disclosed in US. atent application Serial No. 792,685, filed January21, 1969.

The circuit diagram of FIG. 4 clearly shows that the frequency iscontrolled by the actual value U, of the voltage at the input to theinverter. It is thus a dependent variable. The independent variable forvarying the speed of the motor is the nominal value for voltage which isshifted'with the help of the tapping 17.

It is, of course, not absolutely necessary for the frequency and thevoltage to be in linear relationship with each other. Furthermore, itmay be expedient in some cases to change the frequency not merely independence upon voltage but also in dependence upon the motor current.

I claim:

1. A speed and'voltage control system for an alternating current motorsuch as a polyphase induction motor comprising, a voltage regulator, aninverter having inlet terminals connected to and supplied by saidvoltage regulator, said inverter having at least two branches with eachbranch having two controllable rectifiers arranged in series and twofree running diodes in antiparallel relation to said rectifiers, avoltage to frequency converter having at least one condenser and havingthe frequency of the control output pulses thereof dependent upon thevoltage at said condenser, said voltage to frequency converter beingconnected to said voltage regulator and said inverter with saidcondenser having voltage applied thereacross from said voltage regulatorand from said inverter due to motor generator action, a line rectifierconnected to said voltage regulator, and voltage setting means connectedto said voltage regulator for producing selected nominal voltages, saidvoltage regulator having switch means for producing high frequencyimpulses of variable width in dependence on said nominal voltages.

2. A speed and voltage control system according to claim 1 in which saidvoltage to frequency converter includes resistor means connected to saidcondenser and being subject to the voltage at the inlet terminals ofsaid converter, and a unijunction sweep generator connected to saidresistor for controlling the charging of said condenser responsive tothe voltage across said resistor.

k l t i

1. A speed and voltage control system for an alternating current motorsuch as a polyphase induction motor comprising, a voltage regulator, aninverter having inlet terminals connected to and supplied by saidvoltage regulator, said inverter having at least two branches with eachbranch having two controllable rectifiers arranged in series and twofree running diodes in antiparallel relation to said rectifiers, avoltage to frequency converter having at least one condenser and havingthe frequency of the control output pulses thereof dependent upon thevoltage at said condenser, said voltage to frequency converter beingconnected to said voltage regulator and said inverter with saidcondenser having voltage applied thereacross from said voltage regulatorand from said inverter due to motor generator action, a line rectifierconnected to said voltage regulator, and voltage setting means connectedto said voltage regulator for producing selected nominal voltages, saidvoltage regulator having switch means for producing high frequencyimpulses of variable width in dependence on said nominal voltages.
 2. Aspeed and voltage control system according to claim 1 in which saidvoltage to frequency converter includes resistor means connected to saidcondenser and being subject to the voltage at the inlet terminals ofsaid converter, and a unijunction sweep generator connected to saidresistor for controlling the charging of said condenser responsive tothe voltage across said resistor.